Variation in Dorsal Fin Morphology in Common Bottlenose DolphinTursiops truncatus(Cetacea: Delphinidae) Populations from the Southeast Pacific Ocean

Abstract

We succinctly summarize population ID information for common bottlenose dolphins Tursiops truncatus of the Pacific coast of South America, covering four coastal range states. Phylogenetic uncertainties relate primarily to offshore vs. coastal (inshore) ecotypes and biogeographic borders between the five proposed populations (2 offshore, 3 coastal): Colombia-Ecuador Offshore stock (probably = ETP Offshore), Peru-Chile Offshore, Ecuador Coastal, Peru Coastal and an unique community (Pod-R) on the north-central coast of Chile. Main questions concern the extent of gene flow between the offshore stocks at one hand, and with -and between-the three coastal populations at the other hand. Seven cranial characters, four non-metric (separation of occipital condyles, pterygoid bone development, exostosis of periotic) and three metric (tooth width, antorbital process length, maximum width palatines), dorsal fin shape, body stockiness, mt-DNA (control region), habitat, prey composition, parasite load, behaviour and prevalence of some infectious diseases differentiate coastal from offshore forms. 'Pod-R' is the southernmost (29°15'S) and only confirmed coastal form community in Chilean waters, albeit with an offshore (falcate) dorsal fin. Bottlenose dolphins which regularly transit nearshore in the Lagos and Aysén regions (Chilean Patagonia) and occasionally enter deep fjords, present an offshore morphotype. We suggest that two other coastal areas in Chile where bottlenose dolphins have been documented over decades, one a ca. 60 km stretch of coastline centered at Valparaiso/Laguna Verde (33°10'S) in central Chile, and a 190 km coastline around the Mejillones Peninsula (23°10'S) in northern Chile, may not host coastal but offshore form animals. The continental border of the Atacama Trench off northern and northcentral Chile leaves an extremely narrow, steep shelf with nearshore deep water, locally with strong coastal upwelling and increased productivity. This habitat seems to attract oceanic cetaceans, including offshore T. truncatus, sperm whales, large balaenopterids, and other species. The southern distribution range of true coastal morphotype bottlenose dolphins in Pacific South America remains unknown but off Chile distance-to-shore may not be the reliable indicator of ecotype as it is further north in the study region.

How to tell them apart? Discriminating tropical blackfish species using fin and body measurements from photographs taken at sea

“…It is common for cetaceans to have allometric growth, e.g ., common bottlenose dolphin, Tursiops truncatus (Félix et al . ) and killer whale (Clark and Odell ), or a combination of feature‐specific allometric and isometric growth, e.g ., long‐finned pilot whale, Globicephala melas (Sergeant ); Dall's porpoise, Phocoenoides dalli (Amano and Miyazaki ); dusky dolphin, Lagenorhynchus obscurus (Van Waerebeek ); and vaquita, Phocoena sinus (Torre et al . ).…”

“…4, 2019 killer whales, and finally short-finned pilot whales had the least tapering. The latter two ratios are occasionally used to discriminate between coastal and offshore ecotypes of cetaceans (e.g., Félix et al 2018), and highlight differences between these four blackfish species.…”

“…These results indicate either that fin growth in Hawaiian blackfish is isometric where features develop at a rate directly proportional to body size, or else that body and fin features develop at the same rate across all four species, which maintains their interspecies relationships. It is common for cetaceans to have allometric growth, e.g., common bottlenose dolphin, Tursiops truncatus (Félix et al 2018) and killer whale (Clark and Odell 1999), or a combination of feature-specific allometric and isometric growth, e.g., long-finned pilot whale, Globicephala melas (Sergeant 1962); Dall's porpoise, Phocoenoides dalli (Amano and Miyazaki 1993); dusky dolphin, Lagenorhynchus obscurus (Van Waerebeek 1993); and vaquita, Phocoena sinus (Torre et al 2014). It is possible animals were assigned to age classes that could not adequately detect allometric growth rates because they spanned multiple growth phases or did not encompass a growth phase in its entirety.…”

“…Fin and flipper features are adapted to meet habitat or foraging demands, as observed in off-shore and coastal bottlenose dolphin populations (Hersh and Duffield 1990, Morteo et al 2017, Félix et al 2018).…”

Phenotypic variation in dorsal fin morphology of coastal bottlenose dolphins (Tursiops truncatus) off Mexico

“…Morphological variation of the dorsal fin, to some extent, has been used for population and/or species identification (Lang & Pryor, 1966; Aleyev, 1977; Fish, 1998; Weller, 1998; Morteo, Morteo & Rocha-Olivares, 2005; Felix et al, 2017). …”